The invention relates to a hydrostatic drive system.
Known hydrostatic drive systems consist of a variable displacement pump operating on a constant or variable motor. The pivot angle of the pump is proportional to the motor speed. There is a volume flow coupling between the pump as primary unit and the motor as secondary unit. Load and speed changes of the secondary unit induce a pressure reaction.
In contrast thereto, the invention has a drive system with secondary regulation. The primary unit maintains an impressed pressure in a conduit train via a pressure control. During motor operation the secondary unit withdraws the energy necessary for maintaining the desired speed. During pump operation, energy is driven from the load, for example when braking a vehicle or paying out a load, and fed back into the conduit train. Since motor and pump operation are both possible in either direction of rotation, the system has a four-quadrant operating mode. The secondary regulation comprises a pressure coupling between the primary unit and the secondary unit. The secondary unit a predetermined torque as a function of the impressed pressure and the pivot angle of the secondary unit. The speed of the secondary unit is a function of the magnitude of the load and the pivot angle.
To maintain the impressed pressure it is assumed that the operating state of the primary unit is uncoupled from that of the secondary unit by a hydraulic accumulator connected to the conduit train or that the power of the secondary unit is adapted to that of the primary unit. It is also assumed that not only does the pressure in the conduit train not collapse but that the pressure in the conduit does not drop below a specific tolerance band. If however the pressure regulation of the primary unit reacts too slowly, the accumulator is emptied and the secondary unit requires considerable power. There is a danger that, due to the pressure breakdown in the conduit train, the pressure coupling of the drive system will function as a flow coupling. This not only cancels the advantages of the pressure coupling but also makes the control function impossible because the control of the pivot angle of the secondary unit in the pressure-coupled drive system is opposite to that in a flow-coupled drive system.
The invention is therefore directed to solving the problem of maintaining the impressed pressure within a predetermined tolerance band in the conduit train of the drive system and thus maintaining operation of the drive system in pressure coupling.